In an internal combustion engine of a vehicle, for the purpose of correcting variations in the manufacture of components associated with the engine such as fuel supply parts, fuel injection valves, air flow meters, oxygen sensors, fuel pressure regulators, as well as variations caused by deterioration of durability, and for controlling the fuel injection volume to a designed median, there is provided a known air-fuel ratio controller which provides an air-fuel ratio learning control to prevent deterioration of exhaust gases and maintain the driving performance in a satisfactory condition.
In the internal combustion engine there also is provided a known evaporated fuel controller for preventing evaporated fuel from flowing out of a fuel tank. The evaporated fuel controller comprises a canister disposed between an evaporation passage communicating with the interior of the fuel tank and a purge passage communicating with an intake system of the engine, and a purge valve disposed in an intermediate position of the purge passage to control the amount of purge (evaporated fuel) to be fed to the intake system in accordance with the operating condition of the engine.
Examples of such air-fuel ratio controllers which control the air-fuel ratio or control the amount of purge are disclosed in Japanese Patent Laid Open Nos. 7-259610, 7-166936, 5-156988 and 8-240138 and Japanese Patent No. 2545438.
According to the apparatus disclosed in Patent Laid Open No. 7-259610, during execution of learning, a learning end condition is judged on the basis of a deviation between an actual air-fuel ratio and a target air-fuel ratio, and when the learning end condition is established, a purge valve is opened, while if the learning end condition is not established for a predetermined period of time, learning is stopped temporarily and the purge valve is opened forcibly, thereby ensuring the discharge of evaporated gas even upon sticking of a learning value in air-fuel ratio learning.
According to the apparatus disclosed in Patent Laid Open No. 7-166936, a dual oxygen feedback control using a rear oxygen sensor is executed whereby, even in the event there continues a state in which an output signal of a downstream-side oxygen sensor is not inverted, a learning value is forcibly updated into an optimal state of emission once a predetermined state is reached.
According to the apparatus disclosed in Patent Laid Open No. 5-156988, a calculated basic fuel injection volume is corrected using both an air-fuel ratio correction coefficient and a learning value, but only when the air-fuel ratio detected by an air-fuel ratio sensor is within a predetermined range at an inversion timing of a target air-fuel ratio and is inverted following the inversion of the target air-fuel ratio. Then a learning value is detected in accordance with the amount of deviation between the air-fuel ratio detected by the air-fuel ratio sensor and a theoretical air-fuel ratio, thereby preventing the learning value from being affected by disturbance.
Japanese Patent Laid Open No. 8-240138 discloses an air-fuel control in a lean-burn type internal combustion engine in which, at a vehicle speed at which the vehicle nearly stops, there is calculated a purge cut time. This calculated time is subtracted during purge, and when the integrated time has reached a predetermined time or more, a purge gas concentration detection end flag is reset, then after start-up, the inhibition of a theoretical air-fuel ratio feedback control is released and an air-fuel ratio feedback control is forcibly executed even during acceleration to estimate a purge gas concentration, then a shift to a lean driving is judged, thereby preventing the air-fuel ratio from becoming rich during lean driving.
According to the apparatus disclosed in Japanese Patent No. 2545438, there is used a control means for performing an air-fuel ratio feedback control forcibly whereby, when an altitude difference in going up and down a slope has reached a predetermined value continuously, a learning control for the air-fuel ratio is executed by an air-fuel ratio feedback control to compensate for a delay in the air-fuel ratio learning control sufficiently and attain a highly accurate air-fuel control.
In the conventional air-fuel ratio controllers for an internal combustion engine, if the air-fuel ratio learning control is performed during purge control in the evaporated fuel controller, the correction of the air-fuel ratio is not effected properly, thus causing deterioration of exhaust gases and of driving performance. As a result, it sometimes becomes necessary to repeat on-off of purge during the air-fuel ratio learning control.
In such a case, however, if the purge-ON frequency of the canister is increased, the air-fuel ratio learning frequency at purge-OFF becomes smaller. Conversely, if the air-fuel ratio learning frequency is increased, the purge-ON frequency becomes smaller. This is inconvenient. Particularly, when exhaust gases are measured during manufacture in a factory, the air-fuel ratio learning control is scarcely performed, thus resulting in deterioration of exhaust gases. In Patent Laid Open No. 7-259610, when the learning end condition does not exist for a certain period of time, the learning control is stopped and the purge valve is opened forcibly, with the air-fuel ratio learning control being not performed during manufacture in a factory, thus causing a fear of deterioration of exhaust gases.